The present invention relates generally to polymer particles. In particular, the present invention relates to emulsion polymerization processes for forming polymer particles useful for opacifying.
Polymer particles containing voids that impart favorable properties in compositions containing the polymer particles are known in the art. These polymer particles are useful as opacifying agents to supplement, or replace, pigmentary matter. The polymer particles may be used in various applications such as, for example, paints, coatings, inks, sunscreens, and paper manufacture.
Conventional methods for producing these polymer particles entail forming a base-swellable polymer core and encapsulating the polymer core in a polymer sheath. A base is typically added during formation of the polymer particle to swell the polymer core while the polymer sheath is being formed around the polymer core. Upon drying, the swollen polymer core shrinks and a void is created inside the polymer particle.
The polymer sheath is typically plasticized during formation of the polymer core to enhance swelling of the polymer core, which improves the opacifying characteristics of the polymer particle. Conventional methods for plasticizing the polymer sheath typically entail reducing the glass transition temperature of the polymer sheath by either adding solvents or increasing the concentration of free monomer. Solvent-induced plasticization may be disadvantageous, however, since the solvents are volatile organic compounds and, if not removed, permanently reduce the glass transition temperature of the polymer sheath.
Likewise, conventional methods for increasing the concentration of free monomer to plasticize the polymer sheath may also be disadvantageous. These conventional methods include increasing the concentration of free monomer by either increasing the rate of monomer addition or adding an inhibitor to inhibit the free-radical polymerization of the polymer sheath. Increasing the rate of monomer addition may be problematic since it causes increased polymerization and, thus, increased generation of heat, which may be difficult to control in large-scale production applications. Inhibiting free-radical polymerization using an inhibitor may also be problematic since it requires the additional steps of stopping polymerization of the polymer sheath and subsequently reinitiating polymerization, which results the in use of excessive amounts of initiator.